1. Field of the Invention
This invention is concerned with a process for the lubrication of tool surfaces, such as die surfaces, by producing on the tool a film of a high-temperature lubricant.
2. Description of the Prior Art
Graphite containing lubricants are for example known in metal extrusion. At very high temperatures and lengthy deformation process, such as for example in isothermal die forging, such lubricants fail in general, as completely water free graphite has insufficient antifriction properties. Moreover, there is a tendency for a chemical reaction with the material of the die (especially molybdenum based alloys such as TZM) or workpiece. Such a carbon pick-up is however completely undesirable from the point of view of the material properties. Since the workpiece and tool have practically the same temperatures during isothermal high temperature forming operations, the tendency for mutual galling increases and further stricter requirements are placed on the lubricants used, than is the case in conventional extrusion, hot drawing and forging. Consequently lubricants based on oils, soaps, graphite or molybdenum sulphide are unsuitable for the present purposes. It has been attempted to produce a lubricant film on the surface of the tool or that of the piece to be deformed, using boron nitride, mixtures of it with boric acid and graphite and glassy material. These materials are mostly used as suspensions or pastes in organic solvents such as alcohol or toluene or as suspensions in other liquids. The coating of the metal piece with the lubricant is thereby achieved by dipping, smearing or spraying and is followed by a drying process at room temperature or a baking at a higher temperature. Such lubricants and processes are known from several publications. (e.g. T. Watnough, "Development of isothermal forging of titanium centrifugal compressor impeller", AMMRC Report CTR 73 - 19, IIT Research Institute Chicago, May 1973, p.16; U.S. Pat. No. 3,635,068; "Isothermal forging of precision metal powder components", Report AD-780 044, published by NTIS, National Technical Information Service, U.S. Department of Commerce, December 1973, p. 27, 41, 43) The known lubricants and lubricant combinations as well as the corresponding processes using the lubricants are unsatisfactory with respect to the requirements of temperature and pressure resistance, chemical inactivity with respect to the metal surface to be protected and to be lubricated, as well as lubricating power and optimum viscosity. This is apparent from the above mentioned publications (Report AD 780-044 of the NTIS, p. 43, 50, 54, 58; also D. J. Abson, F. J. Gurney, "Heated dies for forging and friction studies on a modified hydraulic forge press", Metals and Materials, December 1973, p. 539).
Furthermore the known processes for producing a lubricating film on the metal surface of interest by dipping, spraying, baking etc. are complicated, expensive and time-consuming. The active component of conventional lubricants, suspended in a liquid, must be applied wet on to the tool. Drying-out or evaporation of the liquid medium must be prevented. If on the other hand the lubricant, eg. boron nitride, is applied in solid form by conventional spraying of a powder, then its adhersion to the metal surface leaves something to be desired. Moreover thick coherent coatings can be made by these conventional methods only with difficulty. Hence there is a definite need for new methods for the production of lubricant films on metallic tools which are continuously exposed to high temperatures. In particular there is a requirement for simple ways and means of providing a coherent lubricant protecting layer on tools whose surface has been roughened by a previous machining operation (turning, milling, planing grinding, polishing, eroding, electrochemical machining etc.) and contains pores.